| Bill Allombert on Sun, 03 Dec 2023 15:31:39 +0100 |
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| Re: PARI/GP pthread questions |
On Sun, Dec 03, 2023 at 03:09:49PM +0100, Karim Belabas wrote: > * Bill Allombert [2023-12-03 13:15]: > [...] > > > 2) > > > Under 3.4.22 nbthreads > > > ... > > > * pthread: number of threads (unlimited, default: number of cores) > > > ... > > > > > > On 16C/32T AMD 7950X CPU I see 3200% CPU in top when starting > > > GP script with parforeach. > > > > > > So should above doc be corrected to > > > > > > "default: #cores * #threads_per_cor" > > > > > > or better to be correct for multi-CPU systems: > > > > When we wrote that, hyperthreading was only used in mainframes. > > It would be much better if the default was the number of cores instead of the number > > of hyperthreads. > > Unfortunately the GNU C library only report the number of CPU threads > > see getconf "_NPROCESSORS_CONF". > > I recommends to set nbthreads to the total number of cores. > > Is there a difference with the simpler 'nproc' (from coreutils) ? from looking at the source code, nproc is actually more complex and still count hyperthreads. > For systems with hyper-threading, one can get the number of physical cores > as follows on my laptop: > > # grep '^cpu cores' /proc/cpuinfo | uniq > cpu cores : 4 This is only work if you have a single CPU! Cheers, -- Bill Allombert Ingénieur de recherche en calcul scientifique ❄ CNRS/IMB UMR 5251